Machines used to break up concrete and asphalt pavement and other hard surfaces have a plurality of tools mounted on a cutting wheel which is forced against the surface to be broken up. Each tool has an elongate steel body at the forward end of which is a tungsten carbide cutting tip. Until the present invention the cutting inserts of such tools have been formed as a unified part. When the wheel rotates, the tools are carried through a circular orbit and the tungsten carbide tips penetrate the hard surface with each tip removing a small amount of material, thereby advancing the cut.
The tools suffer wear as a result of being moved against the hard material being cut and they have to be replaced at regular intervals. Each time the tools are replaced, the machine is taken out of service for a lengthy period of time. Machines used to break up concrete and asphalt roadways are kept in continuous operation through the work day except for when the tools are being replaced, and it is not uncommon to replace the tools in such machines two or three times during a work day. The frequency with which tools have to be replaced and the time consumed during the replacement process therefore reduce the efficiency of the machine and increase the cost of its operation.
The cost of replacement tools and the efficiency with which the tools cut the hard abrasive material also effect the economic efficiency of the machine. To minimize tool costs it desirable that the components of the tool, namely the tool body and the tungsten carbide cutting tip, have comparable endurance to wear. The energy needed to operate the machines, and therefor the cost of operation, increases if the cutting tips become too blunt before the tool body has become sufficiently worn to require replacement. Both the cost of the tool and the cutting efficiency of the tool are important factors in maximizing the efficiency of the tools.
Tool failure can occur as a result of the failure the braze material holding the tungsten carbide tip into the seat at the forward end of the tool body. To prevent failure of the braze and the dislodging of the insert, the hardened inserts of such tools should have a base diameter of about 0.700 inches.
Theoretically, the life of the cutting insert will be increased by providing a larger sized insert, however enlarging the diameter of a currently available one piece tungsten carbide cutting tip will reduce the efficiency of the machine because the tip will rapidly become blunt. Since the tungsten carbide is the most expensive element in the tool, increasing the size of the insert will also increase tool cost. On the other hand, the metal which makes up the tool bodies is subject to wash away causing the tool to fail as aggregate of the hard material cut by the machine erode away the metal of the tool body behind the tungsten carbide tip.
It has long been recognized that the useful life of a tool can be substantially extended by increasing the hardness of the tungsten carbide from which the cutting inserts are made. Efforts to make a tool having a harder insert, however, have not been successful party because harder grades of tungsten carbide are more brittle and tend to fracture, and partly because the harder grades of tungsten carbide are more difficult to manufacture. A harder insert is manufactured by using smaller grain sizes of tungsten carbide particles with a higher concentration of tungsten carbide and a corresponding lower concentration of cobalt. It is the cobalt which cements the sintered tungsten carbide together and to compensate for the reduction of cobalt in the product the particles must also be more uniformly compacted together prior to sintering to reduce the inter granular porosity. If the particles are not uniformly compacted the completed insert will have less dense areas, or porosity, and be subject to failure. Inserts having contoured profiles, such as the insert shown in Ojanen, U.S. Pat. No. 4,497,520, are compacted in dies having contoured walls that are inadequate for uniformly compacting the smaller sized particles needed for a harder insert.
One effort to provide an insert which is made of a harder grade of tungsten carbide which is not subject to fracture is depicted in FIGS. 15-17 of U.S. Pat. No. 5,551,760 to Sollami. The insert depicted in Sollami has a cylindrical core and surrounding the core is an annular collar made of a softer grade of tungsten carbide. The core and the collar are bonded together with a braze joint. The Sollami tip has not reach its expectations because the braze material which holds the parts together does not bond readily to tungsten carbide but does bond well to cobalt, which comprises only a small percentage of the composite material. The problem is exacerbated for the harder grades of tungsten carbide because the cobalt content is a factor in the hardness of the tungsten carbide; hardness being increased as the percentage of cobalt is reduced.
The useful life of the tools could be greatly increased by the provision of a cutting tip as shown by Sollami provided the brazing problems encountered by Sollami could be overcome.
Briefly, the present invention is embodied in a cutting tool for a cutting machine where the tool has a body with a longitudinal axis, a tapered cutting portion symmetric about the axis, a radial flange axially behind the forward cutting portion and a cylindrical shank axially behind the radial flange. The tool body has a seat at the forward end of the cutting portion, and the seat has a generally cylindrical inner wall with a given diameter into which a tungsten carbide insert is brazed.
In accordance with the invention, the hardened insert is made of three components assembled in coaxially relationship. The central portion of the insert is an elongate cylindrical body at the forward end of which is a tapered cutting end. Fitted around the cylindrical body is a non-carbide metal sleeve and fitted around the circumference of the non-carbide metal sleeve is an annual collar made of tungsten carbide. The outer diameter of the collar is sized to fit within the given diameter of the seat at the forward end of the tool body. A braze material retains the sleeve to the central body, retains the collar to the sleeve and retains the assembled insert within the seat at the forward end of the cutting tool.
It has been found that the provision of the metal sleeve between the cylindrical central portion and the annular collar provides a surface which is receptive to liquefied braze material and will draw liquefied braze material between the parts by capillary action so they can bond to the tungsten carbide and retain the parts of the insert in assembled relationship. Another function of the metal sleeve is to more rapidly conduct heat to the assembled parts during brazing. The metal sleeve also offers significant shock absorbing qualities which protects the cylindrical central body of tungsten carbide from fracturing because steel has 15% to 20% elongation properties even when hardened to Rc 43-46.
In a second embodiment of the invention the metal sleeve is machined into the forward end of the tool body. In this embodiment the seat at the forward end of the tool body has two components, a cylindrical central indentation with a conical floor, and surrounding the cylindrical central indentation is an annular indentation. The cylindrical central indentation and the annular indentation are machined into the forward end of the tool body leaving a tubular protrusion standing between them. The cylindrical body of the insert described with respect to the first embodiment and the annular collar of the first embodiment are simultaneously brazed into their respective annular indentation.